The fuel flow in diaphragm type carburetors is dependent on the pressure differential existing between the carburetor venturi and the atmosphere. The venturi pressure depends upon engine design characteristics and operating conditions. Hence if the carburetor is set to deliver the proper quantity of fuel for one operating condition it may not deliver the desired quantity of fuel at all other operating conditions. Thus, for example, if the carburetor is set to provide the proper quantity of fuel for operating under full load open throttle conditions the fuel supply may not be proper when the engine is operating at idle speeds. Another problem is the limitation of engine speed under open throttle no load condition. While various governing devices have been provided these increase the cost weight and complexity of the engine and are frequently unreliable in operation. A further problem occurs in starting the engine under extreme temperature conditions or when the engine is in "vapor lock."